In the field of propulsion systems that can be used for rockets, missiles, spacecrafts, and any type of thrust drives, the different technologies that are currently available for providing the propulsion can be categorized into different propulsion technology families. The families that contain most rockets propulsion systems that are in use today, or planned to be in use in the near future, include chemical rockets, physical powered rockets, electric propulsion rockets, nuclear rockets, and laser/ablative propulsion rockets. These families of propulsion systems have characteristics that are particular to the family.
For example, chemical rockets use chemistry to create an exothermic reaction in the propellant, which causes the propellant to heat up and expand, generating thrust on a vehicle. This heated propellant, a gas or plasma, uses the heat released by the reaction to expand. This expansion pushes against a nozzle attached to the vehicle. This push, acting to separate the reacted propellant from the vehicle is the mechanism of momentum transfer, which provides for the motion of the rocket. Chemical rockets come in many varieties, such as solid rockets, liquid rockets, hybrid rockets, mono/bipropellant rockets, and others. Chemical rockets similarly vary widely in complexity, size, and cost. Chemistry is one of the oldest, and most well understood of the basic sciences behind propulsion mechanisms. This is one reason why chemical rockets are the most prevalent of all rockets in use today. The raw materials that comprise the propellants are also quite abundant. Also, chemical propulsion systems are noted for their large size, high thrust, and average specific impulse (Isp). These aspects, coupled with the energy density of some chemical reactions, allow chemical rockets to provide comparatively cheap means for generating a large amount of thrust, which is necessary to launch from Earth or any other potential launching point.
Physical rockets use the same principle of basic chemical rockets, i.e., having a propellant pushed out of the rocket by its own energy. The difference is that the energy does not come from a chemical reaction, but a physical one. Such reactions include phase changes (liquid to gas) and pressure changes. These reactions tend to be far less energy dense than most reactions used by chemical rockets. Physical rockets, however, tend to have very low thrust, very low Isp, and low efficiencies. For this reason these rockets are mainly used in model rocketry, and rarely used commercially.
Electric propulsion rockets are characterized by powering the ejection of propellants with a power supply that is kept on the vehicle, and not, as opposed to chemical rockets, stored in the propellant itself. To at least some extent, this limits the total momentum transferred to the rocket by the propellant. As opposed to a chemical rocket, that can be any size by simply adding more propellant, an electric propulsion rocket has a maximum amount of propellant it can carry because it has a limit to the mass of the power supply. Thus, the use of an electric propulsion system is determined in part by its power supply, not just its propulsion mechanism.
Electric propulsion rockets vary wildly in their construction, and in their operating principles. They do not have to rely on a chemical reaction for their energy, instead it is often stored in something akin to a battery, and so, they have the freedom to use this electrical potential energy in many ways. The complexities of electromagnetism allow for a large number of possible ways to take an electric potential and use it to transfer momentum to something. In most cases, electric propulsion rockets expel low amounts (low mass) of high temperature gases or plasmas at very high speeds. This results in low thrust and high Isp, both incomparable to chemical rockets, the first worse, the second better. The amount of thrust that typical electric propulsion systems can produce is generally not high enough to be able to launch a rocket from Earth to orbit, though their high Isps allow for low mass fractions (larger payload mass and smaller fuel mass) and thus longer term missions. Though it is not practical to launch this kind of system from the ground, once in space, this kind of system can be used for correcting satellite trajectories, deep space missions for probes, and orbit changing. The use of electric propulsion systems in such situations is quite practical and even well proven.
Nuclear rockets have the option to either carry their power in a battery like device, such as a radioisotope thermoelectric generator, or other nuclear reactor, or to carry their power in their propellant, by ejecting small pellets that are to undergo nuclear fission, fusion, annihilation, or a combination thereof. Essentially, this means that nuclear rockets may invoke the complexities of chemical or electric rockets, or both, to cater to a specific goal. As power supplies, nuclear generators and reactors have the possibility to be hugely more energy dense than other electric propulsion power supplies, but this is at the cost of an increased complexity, and the problem of having to deal with getting rid of the excess heat generated by the nuclear power supplies. If pellets of nuclear fuel are detonated to deliver thrust, then these pellets, having reactions with a much higher energy density than chemical reactions, have the ability to be effective interplanetary engines, and have enough thrust to launch from Earth. Nuclear rockets in general, have high Isp and variable thrust to fit the requirement of the application.
The drawbacks to their implementation are their reasonable complexity, albeit simpler than many electric rockets, heat management, safety when using near Earth or in Earth's atmosphere, environmental issues due to radiation, and animate public concern and distrust. Laser, ablative, and beamed energy propulsion systems have rarely been implemented, and have undergone primarily only small scale laboratory testing. They store their energy somewhere outside of the rocket, beam it to the rocket, and use it there. How these devices use it varies greatly. As such, there is no characteristic thrust or Isp for these types of systems, but what is characteristic is the difficulty in beaming energy to a rocket. These technologies are still in early research stages. They are numerous other type of spacecraft propulsion systems and space access that have been discussed in the literature, but many of these proposals have one, or more often many, technological problems and, therefore, are not ready to be implanted in actual flight hardware.
Regardless of all these existing propulsion system technologies, for decades the field of high-performance spacecraft propulsion system research has seen very little beneficial advancement. There is still a strong need for better propulsion systems.